To better understand heroin injection patterns that fall between abstinence and dependence, we propose a study of low-frequency heroin injectors who are not in methadone treatment. It is generally assumed that physiological dependence and multiple daily injections are the norm for heroin users. Yet our research has revealed that among long-term, street-recruited, out-of-treatment heroin injectors, 17% inject only 1 to 10 times monthly. In the 1970s, Norman Zinberg pioneered the study of what he termed "controlled heroin use" and developed the classic framework of drug, set, and setting (DSS) to examine noncompulsive drug use. Since that time, the hazards of heroin injection have multiplied to include HIV/AIDS, epidemics of viral hepatitis, and rising rates of fatal overdose. A more contemporary understanding of low-frequency heroin injection (low-FHI) is needed to help inform the fields of substance abuse, drug treatment, and infectious disease prevention. We propose to conduct a qualitative cohort study of 50 low-FHI users and, for comparative purposes, 30 high-frequency heroin injectors (daily heroin injectors;"high-FHI"). Specific aims of the study are the following: Aim 1: To explore transitions in heroin injection frequency by examining lifetime drug use history and prospective 1- and 2-year trajectory of drug use, among low-FHI and high-FHI users. Aim 2: Using the framework of DSS, to assess the motivations and context of low-FHI and compare them with high-FHI. Aim 3: To explore the experiences with and the need for drug treatment among low-FHI users. Aim 4: To explore contextual factors that influence risk for HIV, HCV, and overdose among low-FHI users, and compare them with high-FHI users. To achieve these aims, 50 low-FHI and 30 high-FHI users will be recruited into a cohort study using targeted sampling methods. All study participants will undergo in-depth qualitative interviews and administration of the Addiction Severity Index at baseline and 1 year;in addition, half the participants will be interviewed again at 2 years. Data will also be collected monthly from each participant regarding drug use frequency and life events (e.g., entry into treatment, loss of housing, incarceration) in the past month. These monthly data will provide important cues for follow-up interviews regarding changes in heroin use and their context. They will also be used to trigger brief qualitative "change of status" interviews (n=36). To organize and inform our inquiry, we will use an updated conceptual model of Zinberg's classic DSS framework. Qualitative analysis will be conducted using grounded theory methods. Low-FHI is an overlooked area of drug research with substantial implications for prevention and intervention. An in-depth assessment of low-FHI can inform treatment approaches and expand our knowledge base regarding variations in heroin use among urban poor populations.